Power field effect transistor driver circuits are typically used to drive fractional horsepower DC motors and solenoids. Applications requiring the use of low voltage fractional horsepower DC motors and solenoids include electric fuel pumps, anti-skid braking systems, automatic transmission controllers, robotic mechanical controls, small industrial machines, etc. Heretofore, conventional bipolar devices have been used in circuits to drive such loads. The bipolar design is not very useful, however, due to the large base current requirements and high power dissipation of the bipolar transistors. Darlington transistors have been substituted for the conventional bipolar transistors to decrease the base current requirements and thus reducing the power requirements thereto. However, the performance provided by the Darlington transistors suffers from a large forward voltage drop across the devices, especially in low voltage, high current applications. The forward voltage drop reduces the voltage available to the DC motor resulting in a decreased motor speed. The use of Darlington transistors in a motor driver circuit is not very practical in automobile applications having a fixed voltage supply and requiring optimum motor speed. Further, both conventional bipolar devices and Darlington transistors are not readily interfaced to MOS microprocessors for controlling motor speed.
More recently, methods of driving fractional horsepower DC motors with power field effect transistors have been disclosed that overcome the high base drive requirement and forward voltage drop problem. For example, U.S. Pat. No. 4,454,454 discloses an "H" switch circuit employing power MOS field effect transistors which have low power dissipation, low voltage drops, and allows for easy MOS microprocessor interfacing for motor speed control. An "H" switch typically comprises four power field effect transistors capable of driving a fractional horsepower DC motor in forward and reverse directions A high side driver circuit having only one power field effect transistor is used in applications requiring torque in one direction only. The use of "H" switch and high side driver circuits for driving fractional horsepower DC motors in automobile and other critical applications requires proper operation even under environmentally difficult conditions. In the automobile application, for example, large positive voltage spikes may appear at the positive battery terminal due to a battery jump or faulty alternator operation, large negative voltages may appear at the DC motor being driven due to collapsing currents in the inductive windings of the DC motor, and loss of battery ground may be intermittent due to a poor ground supply terminal connection.
Thus, what is needed is a power field effect transistor driver circuit capable of driving fractional horsepower DC motors and other loads while providing protection to a power field effect transistor from negative or positive overvoltages and further preventing spurious operation due to power supply fluctuations or loss of the ground supply terminal connection due to, for example, faulty wiring or vibration.